A surprise swing vote that could seal South Boston Councilor Bill Linehan’s bid for the Council presidency is coming from none other than Michelle Wu, the 28-year-old lawyer who rode to victory in September with strong support from voters of color and liberals.

Since the November election, councilors have been wrangling over the presidency, which brings with it the power to appoint committee chairs and control what matters come before the body for a vote.

Now activists who supported Wu are up in arms that she’s supported Linehan over Roxbury Councilor Tito Jackson, who reportedly has assembled six votes in his bid for the presidency.

Alejandra St. Guillen, executive director of the Latino political organization Oiste?, urged Wu to reconsider her vote.

“Michelle is a progressive,” said St. Guillen. “I know her personally. Linehan’s values do not mirror hers and do not mirror the views of those who supported her. She should reconsider her vote and know that changing her mind would be less of a negative than voting for Linehan.”

Linehan came under fire from Oiste? And other members of a coalition of voting rights activists two years ago during the council’s redistricting process when he attempted to slice several Chinatown precincts from District 2, which he currently represents – a move seen by many as an attempt to shore up the white majority in his district.

He also raised eyebrows with his bid to wrest control of the St. Patrick’s Day breakfast from newly elected 1st Suffolk District state Sen. Linda Dorcena Forry. The annual breakfast has traditionally been hosted by the senator representing South Boston.

Horace Small, executive director of the Union of Minority Neighborhoods said Wu’s pledge to vote for Linehan brings into question whether Wu really is a progressive.

“She sold us on the idea that she’s a progressive, but we don’t know her,” he said. “In the real world, politicians have a track record you can use to gauge whether or not to support them. She has no record.”

Wu, who lives in the South End, worked in communities of color across Massachusetts during Elizabeth Warren’s campaign for the U.S. Senate last year. She place second on the ballot in the November election behind at-large Councilor Ayanna Pressley with strong support in the black community.

“I’m very disappointed,” said political activist Sarah Ann Shaw. “I called her. Linehan has done things that lead me to believe he won’t lead for anybody.”

With calls from angry supporters and an on-line petition from the group JP Progressives, Wu, who’s Twitter hashtag is Wu Train, attempted to Wu ’splain her vote, arguing that Linehan was the best candidate for the job and that supporting him would help move the city forward.

"Over the last year, there has been a lot of talk about a ‘New Boston’ and an ‘Old Boston,’ but I reject the notion that Boston is a City hopelessly divided by neighborhood, income level or political outlook,” Wu wrote in a 400-word letter. “A central theme of my campaign was inclusion, and the only way we can move the whole city forward is by working together – even if that means reaching beyond the confines of what’s easy or comfortable.”

Wu’s growing chorus of critics remains unconvinced.

“Linehan’s role in redistricting and the controversy over the St. Patrick’s Day breakfast were just two of a long list of examples of why any progressive should not support him for City Council president,” said Lydia Lowe, executive director of the Chinese Progressive Association. “I don’t support the idea that he would decentralize power. That’s not been our experience.”

“We need to stay on her,” Small said. “What she has proven is, if there’s a wild card on this council, it’s her. She sold people on the idea that she’s a progressive. She’s not. What does she really stand for?”